The purpose of this study is to evaluate the effects of targeting both the motivation and action phases of behaviour change in a 5-week intervention to increase physical activity (PA) among patients with rheumatoid arthritis (RA) not meeting current PA recommendations. In a randomised controlled trial, a control group—which received a group-based patient education session led by a physical therapist—was compared to a treatment group which received the education session plus a motivational interview from a physical therapist and two self-regulation coaching sessions from a rheumatology nurse. Outcomes included leisure-time PA, days per week with at least 30 min of moderate-intensity PA, self-efficacy and autonomous motivation (cognitions which predict PA initiation and maintenance), disease activity, functional status, depressive symptoms and fatigue. Effects were assessed using mixed models repeated measures. Of the 78 patients randomised, 76 and 67 completed the post-treatment and follow-up assessments, respectively. Significant treatment effects were found for leisure-time PA (p = 0.022), active days/week (p = 0.016), self-efficacy (p = 0.008) and autonomous motivation (p = 0.001). At post-treatment and 6-months follow-up, significantly more treated patients than controls met current PA recommendations. No significant effects were found for disease activity, functional status, depressive symptoms or fatigue. Combining motivation- and action-focused intervention approaches improved PA-related cognitions and led to improved uptake and maintenance of leisure-time PA. However, further research is necessary to identify ways of helping patients with RA transition to—and maintain—more intensive forms of PA which are more likely to improve disease activity and functional status.